Two-stage c{11 {0 dewaxing/deoiling process

ABSTRACT

An improved process for dewaxing and deoiling a petroleum oil stock with a liquid, normally gaseous solvent in which the oil feed is chilled prior to autorefrigeration by dilution with cold recycled filtrate from the final filtration of the wax-oilsolvent mixture.

United States Patent Biribauer et al.

[54] TWO-STAGE C DEWAXING/DEOILING PROCESS [72] Inventors: Frank A. Biribauer, Cranford; James D.

Bushnell, Berkely Heights, both of NJ.

[73] Assignee: Esso Research and Engineering Company,

Linden, NJ.

[22] Filed: Sept. 19, 1969 [21] AppL No.: 859,278

[52] U.S. Cl ..208/3l, 208/35, 208/38 '[5l] Int. Cl ..C10g 43/08 [58] Field of Search ..208/31, 33, 35, 38

[56] References Cited UNlTED STATES PATENTS 2,281,667 5/1942 Bray ..208/35 1 Apr. 25, 1972 2,287,966 6/1942 Brandt ..208/35 2,734,849 2/1956 Gross et a1. .208/31 3,350,297 10/1967 Torobin ..208/3l 3,393,144 7/1968 Button et al. ..208/38 Primary Examiner-Herbert Levine Attorney-Pearlman and Stahl and C. D. Stores ['57] ABSTRACT An improved process for dewaxing and deoiling a petroleum oil stock with a liquid, normally gaseous solvent in which the oil feed is chilled prior to autorefrigeration by dilution with cold recycled filtrate from the final filtration of the wax-oilsolvent mixture.

10 Claims, 2 Drawing Figures START DILUTION TOWER 151 STAGE FILTERS 2nd STAGE 1st STAGE FILTER FILTER FEEDDRUM AND COLD RUNDOWN 27 DRUM 3 BLOW GAS-1 2nd STAGE FILTERS BLOW GAS TWO-STAGE C DEWAXING/DEOILIN G PROCESS BACKGROUND OF THE INVENTION This invention relates to a process for dewaxing petroleum oils, and more particularly to dewaxing petroleum lubricating oil stocks. More particularly, this invention relates to an improved process for crystallizing the wax in a wax-containing petroleum distillate or residual fraction, wherein at least a part of the crystallization is achieved by direct contact with a liquid refrigerant. Still more particularly, this invention relates to a process for dewaxing a hydrocarbon feed whereby said feed is cooled by direct contact with cold solvent prior to autorefrigeration by direct contact with boiling refrigerant. Still more particularly, this invention relates to a process for dewaxing a petroleum oil fraction utilizing a two-stage filtration system and wherein the petroleum feed is cooled utilizing cold final-stage filtrate prior to autorefrigeration.

DESCRIPTION OF THE PRIOR ART It is well known in the art to remove waxy constituents from wax-containing hydrocarbons, particularly from wax-containing petroleum oils, by various methods. These processes generally chill the wax-containing oil to a temperature at which the waxy constituents have crystallized out of solution. The chilled mixture is then handled in a manner to separate the crystallized wax particles from the dewaxed oil by various means, usually by filtration, although sedimentation or centrifugation may be used. Solvent is then recovered from both products by distillation.

The ratio of solvent to oil in such processes is dependent upon the particular oil and solvent employed and can be adjusted to facilitate the easy handling of the wax slurry and to optimize filter rate. Typically, the total volume of solvent circulated is three to six times the volume of oil fed. A wellknown batch-type process, commonly called the propane dewaxing process, employs as dewaxingsolvent, a liquid normally gaseous hydrocarbon such as propane, which is also an autorefrigerant. Sufficient solvent is added to the oil to completely dissolve the wax particles therein. In this process it is necessary to heat the oil-solvent mixture in order to obtain complete miscibility of the wax particles therein. Thus, this mixture is usually heated to about l40l60 F. and then cooled in indirect heat exchange equipment to between about 60 and 100 F., just above the cloud point, prior to autorefrigeration. The heating has been found necessary in order to subsequently obtain a filterable wax slurryupon crystallization of the wax by autorefrigeration of the slurry. In this process, the chilling of the slurry and crystallization of wax, is effected by vaporizing a portion of the liquid diluent, or refrigerant. This is achieved by gradually lowering the pressure in the batch chilling vessel. That is, this pressure reduction causes the volatile, normally gaseous liquid refrigerant to boil, or vaporize. Thus, the main body of liquid supplies the latent heat of vaporization to the refrigerant, and the temperature of the slurry is lowered. The pressure is lowered to a point corresponding to the temperature at which the wax dissolved in the petroleum fraction has crystallized out of solution to the desired extent, whereupon the slurry is filtered.

It is also known in the art to dewax petroleum lubricating oils with ketone or other solvents in a two-stage process with countercurrent use of the solvent. In the first stage, the waxy oil feed is chilled continuously in scraped surface, indirect heat exchange equipment, from about its cloud point down to the filtering temperature. Solvent is preferably added in several increments along the chilling train, the temperature of each increment of solvent being adjusted before its addition, to the stream temperature at its point of addition. A substantial fraction of the total dilution solvent may be added on completion of chilling, at the filtration temperature. The slurry is then filtered. The wax from this first stage filtration is reslurried with additional cold solvent and refiltered in a second stage of filtration to recover additional dewaxed oil and lower the oil content of the wax product. The dewaxed oil product is recovered from the primary filtrate. The secondary filtrate, which is relatively lean in oil, may be re-used without reheating as the final first stage dilution and wax cake wash solvent while a major part of the total recovered solvent is employed as reslurry solvent and wash on the second stage filters. This countercurrent use of solvent reduces the total amount of recovered solvent needed in the process thereby substantially improving operating efficiency. However, in propane dewaxing plants prior to this time, it has not been possible to use cold second stage filtrate effectively in the first stage since most of the first stage propane which is required for dilution and autorefrigeration in the conventional propane process must be added at elevated temperatures. Addition of cold solvent to the warm slurry in the conventional process is known to result in the formation of very fine crystals which would filter poorly.

BRIEF SUMMARY OF THE INVENTION It has now been found that the foregoing deficiencies in propane dewaxing processes can be avoided with the dewaxing process of the present invention. Accordingly, it is an object of this invention to provide a propane dewaxing process in which preheating of the oil-solvent mixture is unnecessary prior to autorefrigeration. A further object of this invention is to provide a process whereby the initial oil solution may be cooled substantially below its cloud point prior to entry into the autorefrigeration chiller. A further object of this invention is to provide a stage-wise propane dewaxing process wherein the filtrate from the final stage may be used to cool the initial oil-solvent solution prior to its entry into the autorefrigerative chillers. Another object of this invention is to provide a multistage propane dewaxing and/or deoiling process wherein the final stage filtrate may be effectively utilized countercurrently to cool and dilute the initial oil solution. A still further object of this invention is to provide a dewaxing process which may be simultaneously used to deoil wax. Other objects will be apparent from the following description.

According to this invention, the foregoing and other objects are accomplished by utilizing the final filtrate obtained in the multi-stage filtration of a-dewaxed oil wax mixture to cool the initial oil feed prior to autorefrigeration thereof.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 and 1A represent a flow diagram of the dewaxing/deoiling process.

DETAILED DESCRIPTION Referring to the drawings, a petroleum distillate fraction is fed from the feed retention drum 1 through lines 2 and 4 into the dilution chilling tower 5.

The oil feed, prior to its entry into the dilution tower 5 through line 4, is at a temperature above that at which the wax contained therein will precipitate, that is, above its cloud point. When necessary, the oil feed may be heated in the heat exchanger 3. The oil feed entering through line 4 will therefore generally be at a temperature of between and I50 F.

The dilution chilling tower 5 is a vertical tower which will allow the oil feed to pass therethrough and which contains injection lines 49, for allowing the addition of cold solvent at from -45 to 0 F., incrementally along the height thereof. The contacting of the oil with the cold solvent along the height of the tower thus cools the oil feed at a rate of between 2 and 10 F. per minute, so that'the oil-solvent mixture leaving the dilution chiller tower 5 through line 6 is at from'0 to about +40 F. The pressure in the dilution chilling tower 5 will be such that the solvent therein will be maintained in a liquid phase.

As is shown in co-pending U.S. Ser. No. 666,268, filed Sept. 8, 1967, and now abandoned it is critical in the dilution/chilling tower that the degree of agitation in the stirred verticaltower be sufficient to permit substantially instantaneous'mixing'of the oil and solvent. The prechilled solvent is Chemical Engineers Handbook, 3rd, page 1,224 McGraw- Hill, New York, (1959)], N,,,, which is defined by the equation:

where L agitator diameter, feet;- 7 liquid density, pound/foot; N agitator speed, rps.; up liquid viscosity, pound/foobsecond; is over about 1,000, preferably at least 10,000, well into the so called turbulent range, and the dimensionless ratio of cooling tower diameter to agitator diameter is between about 1.5/1 and about /1. A turbine type agitator is preferred; however, other types of agitators such as propeller and disc may be used.

The tower is divided into a plurality of mixing (cooling) stages by horizontal circular plates and/or shallow dished heads which restrict flow between the stages to an annular opening. In general, the dimensionless ratio of the cross sectional area of the restricted flow opening to the cross section of the tower will be between about 1/50 and about l/IOO. Each mixing zone is provided'with an agitator and one or more cold solvent inlets. The cooling tower mixing zones may or may not be baffled, but baffled mixing zones are preferred.

As cooling progresses in the dilution chilling phase of the process, the ratio of solvent to feed in the mix increases. Conversely, in the subsequent autorefrigeration chilling phase, the dilution ratio decreases due to evaporation of solvent. The proportion of total chilling done ineach phase of the overall chilling is preferably adjusted so that the amount of cold solvent injected indilution chilling, less the amount evaporated in autorefrigerative chilling, will result in a slurry at the dilution level desired for filtration, when the desired filtration temperature is reached. This proportion will vary with the dilution level desired on a particular feedstock, and other factors, but generally permits about two thirds of the total chilling to be done in the dilution chilling tower. The partially chilled oil-solvent-wax slurry thus formed flows by line 6 to batch feed drum 7, from which it is fed alternately into one or the other of batch autorefrigerative chillers 11.

Two of these batch chillers are provided, together with batch feed drum 7 and first stage filter feed drum 19, to permit continuous output of partially chilled slurry from dilution tower 5 and to provide a continuous supply of fully chilled slurry to the first stage filters 23. One of the batch chillers 11 goes through a chilling cycle while the other empties into drum 19. The emptied drum is then warmed up by repressuring with hot compressor discharge gas via line 85, and recharged (about half full) from drum 7 via line 8 and line 16. Chilling is accomplished by releasing gas at a controlled rate from chiller 11 to the propane compressor suction 72 via lines 12, 13, l4, 15, 21 and 40, knock out drum 70 and line 71. The pressure on the chiller is gradually reduced and the autorefrigerant vaporizes, progressively chilling the oil-solventwax slurry in chiller 11 further and erystallizing additional wax from solution. Typically, batch autorefrigerative chilling is continued until the pressure approaches atmospheric and the temperature of the slurry is 30 to -40 F. Thereupon, the liquid slurry or dewaxed oil-wax-solvent mixture is withdrawn from the chiller through lines 17 and 18 to first stage filter feed drum 19.

The liquid-solids separation may be carried out utilizing several means known in the art, such as filters, centrifuges or settling, with filtration in a continuous rotary type filter being preferred. The wax from the first stage separation typically contains between 10 and 30 wt. percent dewaxed oil (expressed on a solvent free basis) and, as removed from the filter, includes 50 to 70 wt. percent solvent. The filtrate phase also contains between 50 and 70 wt. percent solvent, the remainder being dewaxed oil.

Solvent is removed from the liquid phase or filtrate by flashing in high and low pressure flash towers and finally in an explosivity corrector" where it is subjected to vacuum, resulting in a liquid dewaxed oil containing a negligible amount of solvent, and having a final pour point of between 0 and +15 F. and a final cloud point of from +5 to +20 F. The solid or wax phase withdrawn from the first stage separation means is further purified by the addition of further cold solvent thereto under agitated conditions and then a further separation step, again resulting in liquid and solid phases. The liquid phase or filtrate from the second stage separation typically contains from 2 to 10 wt. percent dewaxed oil, the remainder being sol vent, and the wax-oil content (solvent free basis) is reduced to between 0.1 and 5 wt. percent. This process may be continued for as many stages as is desired. Each such separation stage will result in the formation of two phases, a wax phase of reduced oil content, and a liquid phase consisting mostly of cold solvent. The cold filtrate, or liquid phase, from the final such stage, consisting mainly of solvent, but with some oil therein may then be recycled through line 48 and injection lines 49 for use in the dilution chilling tower, to cool the original oil solution as described previously.

Solvent is separated from the final wax phase by flashing in a flash tower typically maintained at a temperature of 325 to 450 F. and a pressure of from 150 to 300 psig, and thus producing a final wax product containing from 0.1 to 5 wt. percent dewaxed oil.

The petroleum distillate fractions employed in this process boil within the range of 600 to l,100 F., preferably from 725 to l,040 F. Typical such fractions from waxy crudes have an initial wax content of from about 10 to 20 wt. percent, and initial pour and cloud points of between about and 130 F. Residual petroleum fractions, boiling above about l,000 F., from which the asphalt has been removed, are also satisfactory feedstocks for this process. Such stocks typically contain 10-20 percent wax and have pour points in the range of 80-150 F. These petroleum oil fractions may come from many sources, such as the paraffinic crude obtained from Arabia, Kuwait, the Texas Panhandle, North Louisiana, Western Canada, etc.

The solvent refrigerant employed in conjunction with this invention is a liquid, normally gaseous hydrocarbon from C through C such as propane, propylene, ethane, ethylene, butanes, butylenes and mixtures thereof. Propane and propylene are preferred. These solvents, being very fluid at low tempera tures, reduce the viscosity of even the heaviest oils to such an extent that satisfactory filter rates and yields can be obtained at the low temperatures necessary to make low pour point oils, provided that chilling conditions are maintained which will generate wax crystals of satisfactory size and form.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring again to the FIG., a petroleum oil fraction boiling, for example, between 725 and 815 F., containing approximately 15 wt. percent wax, and having initial pour and cloud points of F. and F., respectively, is fed from the feed retention drum 1 through line 2 to a heat exchanger 3 wherein said oil is heated, if necessary, to a temperature above its congealing point, preferably between 90 and F. This oil is fed through line 4 into the dilution chilling tower 5.

Within the dilution chilling tower 5 the oil is mixed incrementally with cold filtrate from the second stage filtration operation. This filtrate, largely propane, is at about 30 F., and is injected at a number of points along the length of the dilution chilling tower at a rate such that the oil is cooled at between 2 and 5 F. per minute as it passes therethrough. This tower is operated under turbulent conditions such that substantially instantaneous mixing of the added solvent is accomplished therein.

The total propane injected through lines 49 into the dilution chilling tower 5 amounts to 2.5-3.5 volumes per volume of waxy oil feed, and chills the mixture down to a temperature between +15 and +30 F., thereby crystallizing out between 50 and 90 percent of the initial wax content of the oil which is to be removed. By the use of the agitation described, easily handled wax crystals are formed in the dilution chilling tower 5 and with the slurry pass through line 6 into the batch feed drum 7. The partially chilled dewaxed oil-wax propane mixture from the batch feed drum passes through lines 8, l and 16 into one of the chillers 11, in which it is batch chilled by evaporation of propane as previously described, to a final temperature of from 20 to 40 F. The remainder of the wax to be removed is thereby crystallized, forming the final waxdewaxed oil-propane slurry. The vaporized propane passes overhead through lines l2, l3, l4, 15, 21 and finally through line 40, knock out drum 70 and line 71 for recompression, while the liquid slurry passes through lines 16, 17 and 18 into the first stage filter feed drum 19. The slurry in drum 19 has approximately the following composition; 8 wt. percent wax, 44 wt. percent dewaxed oil, and 48 wt. percent propane. This slurry passes through line 22 and into the first stage filter 23, for separation of the liquid and solid phases.

The first stage filter 23, is a rotary-type filter operating at substantially the final chilling temperature. Liquid is drawn through the filter cloth on the rotating drum, and a wax cake forms on the cloth. Liquid propane from line 24 is sprayed on the wax cake and drawn through to separate additional oil from the wax. Propane vapor at from 0 to 50 F. and from 10 to 25 psig is used as a blow gas to loosen the wax cake from the filter cloth for discharge. Additional propane is used to transport the wax. The clarified liquid phase withdrawn from the filter, consisting of about 47 wt. percent dewaxed oil and 53 wt. percent propane, passes through line 25 into the first stage filter rundown drum 26. It is then pumped through pump 31 and line 32, heat exchange means 33 and line 41. A minor part enters the dewaxed oil high pressure flash tower 96 through line 93 as reflux while the major part goes through line 42, heating means 94 and line 95 to the tower.

The dewaxed oil-high pressure flash tower 96 is maintained in this example at a pressure of 235 psig and a maximum temperature of 320 F. It contains three reflux stages so that propane may be flashed overhead through line 97 at about 140 F. and dewaxed oil may be withdrawn therefrom through line 100 at 320 F., containing only about wt. percent propane. Additional propane is removed in the dewaxed oil low pressure flash tower 101, maintained at about 320 F. and 5 psig. so that the dewaxed oil removed therefrom through line 102 contains less than 0.5 wt. percent propane. The additional propane flashed overhead in the dewaxed oil-low pressure flash tower 101 passes through line 75 into the knockout drum 70 prior to entering into two stage propane compressor 72-77. The dewaxed oil then passes through explosivity corrector and drier 103 wherein essentially complete removal of residual propane is accomplished by application of vacuum through line 104, pump 105, and line 106, into a heat exchanger 107, where its temperature is lowered to about 150 F., prior to withdrawal through line 108. The final dewaxed oil product has pour and cloud points of 0 to +1 5 F. and +5 to +20 F., respectively.

The wax withdrawn from the first stage filter 23 through line 27, slurried with additional cold solvent, is passed by pump 28 through line 29 into second stage filter feed drum 34. Additional cold solvent is added to drum 34 by line 35 and the whole agitated by agitator 36. A bubble cap plate 34 is provided in this drum to allow separation of liquid and vapor phases. Some liquid propane collects on top of the plate and is withdrawn through line 110 and is discharged into line 24 which furnishes propane spray to filters 23 and 43. The wax slurry settles to the bottom of drum 34 and is withdrawn through line 42 to filter 43. Gaseous propane is removed through line 37 for recycle to the system. Filter 43 is operated in the same manner as first stage filter 23, and may be maintained at the same filtration temperature. The filtrate is withdrawn from the filter through line 44 to the second stage filter rundown drum 45 from which it passes through line 46, pump 47, line 48 and injection line 49 for recycle back to the dilution chilling tower 5.

The wax phase, slurried with as much solvent as necessary for handling, is transported from the filter through line 50, pumps 51 and 53, lines 52 and 54, heat exchange means 55 and heating means 58 to wax high pressure flash drum 56. This flash drum 56 is maintained at a temperature of 340 F. and a pressure of 235 psig. Propane vapors recovered from this drum are cooled and condensed by heat exchange means 55, passed through calcium chloride (CaCl drier 62 and return to propane storage drum 89. The wax product from the drum still contains about 0.3 wt. percent solvents, which could be further reduced by a low pressure flash step (not shown) if desired. The oil content of the wax, in this instance, is reduced to l-2 percent.

EXAMPLE 1 An 8451,040 F. cut from Arabian light crude is dilution chilled in a multi-stage tower from F. to +14 F using 2.9 volumes of 40 F. second stage filtrate per volume of oil feed. This liquid is injected into 16 vigorously stirred stages with the amount of the cold stream to each stage adjusted so that the temperature drop from stage to stage is uniform. The tower is sized for an oil residence time corresponding to an average chilling rate of 4 F./minute. The +l4 F. slurry leaving the dilution chiller is chilled further by batch autorefrigeration (vaporization of part of the propane at a controlled rate), giving a final 1st stage slurry containing 2.0 volumes of propane per volume of oil feed, at 40 F. This slurry is charged to the first stage filters, along with 0.7 volumes of recovered, chilled propane which is applied to wash the wax cake and to transport the wax from the filter. The first stage wax is slurried with 2.2 volumes/vol. feed of additional recovered chilled propane and filtered on the second stage filters, to which an additional 0.7 volumes of fresh cold propane is applied. The filtrate from the second stage filters is recycled to the dilution chiller tower as described above. The filtrate from the first stage filtration and the wax from the second stage filtration are freed of solvent by conventional means and recovered as products.

EXAMPLE ll The following table compares the overall results which can be achieved by the process of this invention, with those obtained by conventional single stage or two stage propane dewaxing when processing a l,040 F.+ deasphalted oil prepared from light Arabian crude to make a +l0 F. pour dewaxed oil SSU/210 F. bright stock).

This Conventional invention- One Stage Two Stage two stage Filtration Filtration Filtration Dewaxed oil yieldvol.% 80 84 84 Wax Oil content-wt.% 25 3-5 3-5 Total recovered solvent required- V/V/Feed 5.2 6.2 5.0 Relative Refrigeration Requirement 1.0 1.1 0.6

Since solvent recovery and refrigeration together represent roughly half of total dewaxing plant investment and operating cost, the savings indicated above represent a large economic advantage for the claimed invention. These savings result from:

1. Lower feed temperature at start of chilling.

2. More opportunity for refrigeration recovery by heat exchange.

3. Effective countercurrent use of solvent without reheatmg.

4. Refrigeration inefficiency inherent in the batch chilling process is minimized.

We claim:

1. A process for dewaxing a wax-containing petroleum oil feedstock and deoiling the wax thus obtained, comprising introducing said feedstock into a cooling zone divided into a plurality of stages, introducing into said cooling zone a liquid, normally gaseous, solvent at a plurality of points along said cooling zone, maintaining a high degree of agitation in at least a portion of said stages so as to provide substantially instantaneous mixing of said solvent and said oil, cooling said oil as it progresses through said chilling zone thereby precipitating at least a portion of the wax contained in said oil and forming a solvent-oil-wax slurry, vaporizing at least a portion of said solvent from said slurry so that a further portion of said wax is precipitated, separating a dewaxed oil and an oil-containing wax from said slurry, removing at least a portion of said oil from said oil-containing wax by mixing therewith solvent thereby forming an oil-solvent mixture containing said wax, separating a deoiled wax from said mixture by stagewise filtration thereof, recovering a final stage filtrate comprising an oilsolvent mixture and recycling at least a portion of said oil-solvent mixture of said final stage filtrate to said cooling zone for use as solvent therein.

2. The process of claim 1 wherein said stagewise filtration comprises 2 stages.

3. The process of claim 1 wherein said solvent comprises propane.

4. The process of claim 1 wherein the solvent-oil wax slurry is withdrawn from said cooling zone at a temperature of from about to +40 F.

5. The process of claim 1 wherein said liquid, normally gaseous, solvent is a C to C hydrocarbon or mixtures thereof.

6. The process of claim 5 wherein said solvent comprises propylene.

7. A process for dewaxing a wax-containing petroleum oil fraction and deoiling the wax thus obtained, comprising introducing said fraction into a cooling zone divided into a plurality of stages, introducing a solvent selected from the group consisting of C C. hydrocarbons and mixtures thereof, into said coolingzone at a plurality of points along said cooling zone, maintaining a high degree of agitation in each of said stages so as to provide substantially instantaneous mixing of said oil and said solvent, cooling said oil as it progresses through said cooling zone at a cooling rate of from about 2 to 10 F ./minute, thereby precipitating at least a portion of the wax contained in said oil and forming a solvent-oil-wax slurry, vaporizing at least a portion of said solvent from said slurry thereby precipitating a further portion of said wax, separating a dewaxed oil and an oil-containing wax from said slurry, removing at least a portion of said oil from said oil-containing wax by mixing therewith solvent thereby forming an oil-solvent mixture containing said wax, separating a deoiled wax from said mixture by stagewise filtration thereof, recovering a final stage filtrate comprising an oil-solvent mixture and recycling at least a portion of said oil-solvent mixture of said final stage filtrate to said cooling zone for use as solvent therein.

8. The process of claim 7 wherein said C -C hydrocarbon comprises propylene.

9. The process of claim 7 wherein said C -C hydrocarbon comprises propane.

10. The process of claim 7 wherein said solvent-oil-wax slurry is withdrawn from said cooling zone at a temperature of from about 0 to 40 F. 

2. The process of claim 1 wherein said stagewise filtration comprises 2 stages.
 3. The process of claim 1 wherein said solvent comprises propane.
 4. The process of claim 1 wherein the solvent-oil wax slurry is withdrawn from said cooling zone at a temperature of from about 0* to +40* F.
 5. The process of claim 1 wherein said liquid, normally gaseous, solvent is a C2 to C4 hydrocarbon or mixtures thereof.
 6. The process of claim 5 wherein said solvent comprises propylene.
 7. A process for dewaxing a wax-containing petroleum oil fraction and deoiling the wax thus obtained, comprising introducing said fraction into a cooling zone divided into a plurality of stages, introducing a solvent selected from the group consisting of C2-C4 hydrocarbons and mixtures thereof, into said cooling zone at a plurality of points along said cooling zone, maintaining a high degree of agitation in each of said stages so as to provide substantially instantaneous mixing of said oil and said solvent, cooling said oil as it progresses through said cooling zone at a cooling rate of from about 2* to 10* F./minute, thereby precipitating at least a portion of the wax contained in said oil and forming a solvent-oil-wax slurry, vaporizing at least a portion of said solvent from said slurry thereby precipitating a further portion of said wax, separating a dewaxed oil and an oil-containing wax from said slurry, removing at least a portion of said oil from said oil-containing wax by mixing therewith solvent thereby forming an oil-solvent mixture containing said wax, separating a deoiled wax from said mixture by stagewise filtration thereof, recovering a final stage filtrate comprising an oil-solvent mixture and recycling at least a portion of said oil-solvent mixture of said final stage filtrate to said cooling zone for use as solvent therein.
 8. The process of claim 7 wherein said C2-C4 hydrocarbon comprises propylene.
 9. The process of claim 7 wherein said C2-C4 hydrocarbon comprises propane.
 10. The process of claim 7 wherein said solvent-oil-wax slurry is withdrawn from said cooling zone at a temperature of from about 0* to 40* F. 